Grafting of high and low molecular weight polymers has been reported often in the literature, see for example, the polymer text book "Polymer Chemistry" by M. P. Stevens (Addison-Wesley), 1975, PP196-202. Maleation is one type of grafting. For example, maleation of polypropylenes progresses easily to higher acid numbers with free radical initiation (see U.S. Pat. Nos. 3,414,551, 3,480,580, 3,481,910, 3,642,722, 3,746,676, 3,932,368 and 4,613,679). The molecular weight of the products of such reactions are lower than that of the starting polypropylene due to accompanying degradation reactions. On the other hand, direct maleation of polyethylenes results in cross-linking which increases the molecular weight of the polymer, see for example, "Journal of Applied Polymer Science", 44, 1941, N. G. Gaylord et al (1992); and U.S. Pat. Nos. 4,026,967, 4,028,436, 4,031,062, 4,071,494, 4,218,263, 4,315,863, 4,347,341, 4,358,564, 4,376,855, 4,506,056, 4,632,962, 4,780,228, 4,987,190, and 5,021,510]. Free radical initiated maleation of polypropylenes and polyethylenes in continuous processes is limited to very low levels of grafting and low acid numbers such as below 4percent grafting. Grafted polyolefins are very suitable for use as compatibilizing agents and adhesion promoters between polymers and fillers. An increased acid number and percent grafting of the polyolefins would provide polyolefins with improved properties such as substrate wetting and adhesion and would even make some polyolefins emulsifiable, rendering them useful in emulsions. However, the higher levels of grafting is difficult to achieve due to the flashing of some of the components and/or crosslinking of the polyolefin in the extruder.
U.S. Pat. No. 4,762,890 discloses the grafting of polymers in a twin screw extruder. This technique is effective for high molecular weight polymers; however, the grafting is only up to 2 weight percent. U.S. Pat. No. 4,639,495 discloses the grafting using peroxide initiators however, the percent grafting is only obtainable up to 3 weight percent.
Efficiently grafting high amounts of comonomer onto polyolefins, producing higher acid numbers, have been attempted but only in batch processes with less than perfect results. Conversion of the batchwise process to a commercial continuous extruder based process has not, heretofore, been practical. However, for a polyolefin grafting process to be commercial and cost effective the process should be continuous. It would, therefore, be very desirable to be able to continuously and efficiently produce highly grafted polyolefins of high acid number.